二十四种设计模式：提供者模式(Provider Pattern)

提供者模式(Provider Pattern)

介绍
为一个API进行定义和实现的分离。

示例
有一个Message实体类，对它的操作有Insert()和Get()方法，持久化数据在SqlServer数据库中或Xml文件里。根据配置文件中的配置来决定数据持久化方案是使用SqlServer数据库还是Xml文件。

　　MessageModel

using System;

namespace Pattern.Provider
{
    /// <summary>
    /// Message实体类
    /// </summary>
    public class MessageModel
    {
        /// <summary>
        /// 构造函数
        /// </summary>
        /// <param name="msg">Message内容</param>
        /// <param name="pt">Message发布时间</param>
        public MessageModel(string msg, DateTime pt)
        {
            this._message = msg;
            this._publishTime = pt;
        }

        private string _message;
        /// <summary>
        /// Message内容
        /// </summary>
        public string Message
        {
            get { return _message; }
            set { _message = value; }
        }

        private DateTime _publishTime;
        /// <summary>
        /// Message发布时间
        /// </summary>
        public DateTime PublishTime
        {
            get { return _publishTime; }
            set { _publishTime = value; }
        }
    }
}

　　MessageProvider

using System.Configuration.Provider;
using System.Collections.Generic;

namespace Pattern.Provider
{
    /// <summary>
    /// 操作Message抽象类
    /// </summary>
    public abstract class MessageProvider : ProviderBase
    {
        /// <summary>
        /// 插入Message
        /// </summary>
        /// <param name="mm">Message实体对象</param>
        /// <returns></returns>
        public abstract bool Insert(MessageModel mm);

        /// <summary>
        /// 获得Message
        /// </summary>
        /// <returns></returns>
        public abstract List<MessageModel> Get();
    }
}

　　SqlMessageProvider

using System;
using System.Collections.Specialized;
using System.Collections.Generic;

using System.Configuration.Provider;
using System.Configuration;

namespace Pattern.Provider
{
    /// <summary>
    /// Sql方式操作Message
    /// </summary>
    public class SqlMessageProvider : MessageProvider
    {
        private string _connectionString;

        /// <summary>
        /// 插入Message
        /// </summary>
        /// <param name="mm">Message实体对象</param>
        /// <returns></returns>
        public override bool Insert(MessageModel mm)
        {
            // 代码略
            return true;
        }

        /// <summary>
        /// 获取Message
        /// </summary>
        /// <returns></returns>
        public override List<MessageModel> Get()
        {
            List<MessageModel> l = new List<MessageModel>();
            l.Add(new MessageModel("SQL方式，连接字符串是" + this._connectionString, DateTime.Now));

            return l;
        }

        /// <summary>
        /// 初始化提供程序。
        /// </summary>
        /// <param name="name">该提供程序的友好名称。</param>
        /// <param name="config">名称/值对的集合，表示在配置中为该提供程序指定的、提供程序特定的属性。</param>
        public override void Initialize(string name, NameValueCollection config)
        {
            if (string.IsNullOrEmpty(name))
                name = "MessageProvider";

            if (null == config)
                throw new ArgumentException("config参数不能为null");

            if (string.IsNullOrEmpty(config["description"]))
            {
                config.Remove("description");
                config.Add("description", "SqlServer操作Message");
            }

            base.Initialize(name, config);

            string temp = config["connectionStringName"];
            if (temp == null || temp.Length < 1)
                throw new ProviderException("connectionStringName属性缺少或为空");

            _connectionString = ConfigurationManager.ConnectionStrings[temp].ConnectionString;
            if (_connectionString == null || _connectionString.Length < 1)
            {
                throw new ProviderException("没找到'" + temp + "'所指的连接字符串，或所指连接字符串为空");
            }

            config.Remove("connectionStringName");
        }
    }
}

　　XmlMessageProvider

using System;
using System.Collections.Specialized;
using System.Collections.Generic;

using System.Configuration.Provider;
using System.Configuration;

namespace Pattern.Provider
{
    /// <summary>
    /// Xmll方式操作Message
    /// </summary>
    public class XmlMessageProvider : MessageProvider
    {
        private string _connectionString;

        /// <summary>
        /// 插入Message
        /// </summary>
        /// <param name="mm">Message实体对象</param>
        /// <returns></returns>
        public override bool Insert(MessageModel mm)
        {
            // 代码略
            return true;
        }

        /// <summary>
        /// 获取Message
        /// </summary>
        /// <returns></returns>
        public override List<MessageModel> Get()
        {
            List<MessageModel> l = new List<MessageModel>();
            l.Add(new MessageModel("XML方式，连接字符串是" + this._connectionString, DateTime.Now));

            return l;
        }

        /// <summary>
        /// 初始化提供程序。
        /// </summary>
        /// <param name="name">该提供程序的友好名称。</param>
        /// <param name="config">名称/值对的集合，表示在配置中为该提供程序指定的、提供程序特定的属性。</param>
        public override void Initialize(string name, NameValueCollection config)
        {
            if (string.IsNullOrEmpty(name))
                name = "MessageProvider";

            if (null == config)
                throw new ArgumentException("config参数不能为null");

            if (string.IsNullOrEmpty(config["description"]))
            {
                config.Remove("description");
                config.Add("description", "XML操作Message");
            }

            base.Initialize(name, config);

            string temp = config["connectionStringName"];
            if (temp == null || temp.Length < 1)
                throw new ProviderException("connectionStringName属性缺少或为空");

            _connectionString = ConfigurationManager.ConnectionStrings[temp].ConnectionString;
            if (_connectionString == null || _connectionString.Length < 1)
            {
                throw new ProviderException("没找到'" + temp + "'所指的连接字符串，或所指连接字符串为空");
            }

            config.Remove("connectionStringName");
        }
    }
}

　　MessageProviderCollection

using System.Configuration.Provider;
using System;

namespace Pattern.Provider
{
    /// <summary>
    /// Message的Provider集合类
    /// </summary>
    public class MessageProviderCollection : ProviderCollection
    {
        /// <summary>
        /// 向集合中添加提供程序。
        /// </summary>
        /// <param name="provider">要添加的提供程序。</param>
        public override void Add(ProviderBase provider)
        {
            if (provider == null)
                throw new ArgumentNullException("provider参数不能为null");

            if (!(provider is MessageProvider))
                throw new ArgumentException("provider参数类型必须是MessageProvider.");

            base.Add(provider);
        }
    }
}

　　MessageProviderConfigurationSection

using System.Configuration;

namespace Pattern.Provider
{
    /// <summary>
    /// Message的Provider的配置
    /// </summary>
    public class MessageProviderConfigurationSection : ConfigurationSection
    {
        private readonly ConfigurationProperty _defaultProvider;
        private readonly ConfigurationProperty _providers;
        private ConfigurationPropertyCollection _properties;
        
        /// <summary>
        /// 构造函数
        /// </summary>
        public MessageProviderConfigurationSection()
        {
            _defaultProvider = new ConfigurationProperty("defaultProvider", typeof(string), null);
            _providers = new ConfigurationProperty("providers", typeof(ProviderSettingsCollection), null);
            _properties = new ConfigurationPropertyCollection();

            _properties.Add(_providers);
            _properties.Add(_defaultProvider);
        }

        /// <summary>
        /// Message的默认的Provider
        /// </summary>
        [ConfigurationProperty("defaultProvider")]
        public string DefaultProvider
        {
            get { return (string)base[_defaultProvider]; }
            set { base[_defaultProvider] = value; }
        }

        /// <summary>
        /// Message的所有的Provider集合
        /// </summary>
        [ConfigurationProperty("providers", DefaultValue = "SqlMessageProvider")]
        [StringValidator(MinLength = 1)]
        public ProviderSettingsCollection Providers
        {
            get { return (ProviderSettingsCollection)base[_providers]; }
        }

        /// <summary>
        /// Message的Provider的属性集合
        /// </summary>
        protected override ConfigurationPropertyCollection Properties
        {
            get { return _properties; }
        }
    }
}

　　Message

using System;
using System.Collections.Generic;
using System.Configuration;
using System.Web.Configuration;

namespace Pattern.Provider
{
    /// <summary>
    /// 暴露给客户端用的Message的类（Context）
    /// </summary>
    public class Message
    {
        private static bool m_isInitialized = false;
        private static MessageProviderCollection _providers = null;
        private static MessageProvider _provider = null;

        /// <summary>
        /// 静态构造函数，初始化
        /// </summary>
        static Message()
        {
            Initialize();
        }

        /// <summary>
        /// 插入信息
        /// </summary>
        /// <param name="mm">Message实体对象</param>
        /// <returns></returns>
        public static bool Insert(MessageModel mm)
        {
            return _provider.Insert(mm);
        }

        /// <summary>
        /// 获取信息
        /// </summary>
        /// <returns></returns>
        public static List<MessageModel> Get()
        {
            return _provider.Get();
        }

        private static void Initialize()
        {
            try
            {
                MessageProviderConfigurationSection messageConfig = null;

                if (!m_isInitialized)
                {

                    // 找到配置文件中“MessageProvider”节点
                    messageConfig = (MessageProviderConfigurationSection)ConfigurationManager.GetSection("MessageProvider");

                    if (messageConfig == null)
                        throw new ConfigurationErrorsException("在配置文件中没找到“MessageProvider”节点");

                    _providers = new MessageProviderCollection();

                    // 使用System.Web.Configuration.ProvidersHelper类调用每个Provider的Initialize()方法
                    ProvidersHelper.InstantiateProviders(messageConfig.Providers, _providers, typeof(MessageProvider));

                    // 所用的Provider为配置中默认的Provider
                    _provider = _providers[messageConfig.DefaultProvider] as MessageProvider;

                    m_isInitialized = true;

                }
            }
            catch (Exception ex)
            {
                string msg = ex.Message;
                throw new Exception(msg);
            }
        }

        private static MessageProvider Provider
        {
            get
            {
                return _provider;
            }
        }

        private static MessageProviderCollection Providers
        {
            get
            {
                return _providers;
            }
        }
    }
}

　　Web.config

<?xml version="1.0" encoding="utf-8"?>
<configuration>
  <configSections>
    <section name="MessageProvider" type="Pattern.Provider.MessageProviderConfigurationSection, Pattern.Provider" />
  </configSections>
  <MessageProvider defaultProvider="SqlMessageProvider">
    <providers>
      <add name="XmlMessageProvider" type="Pattern.Provider.XmlMessageProvider, Pattern.Provider" connectionStringName="XmlConnection" />
      <add name="SqlMessageProvider" type="Pattern.Provider.SqlMessageProvider, Pattern.Provider" connectionStringName="SqlConnection" />
    </providers>
  </MessageProvider>
  <connectionStrings>
    <add name="SqlConnection" connectionString="server=.;database=db;uid=sa;pwd=sa" />
    <add name="XmlConnection" connectionString="XmlPath" />
  </connectionStrings>
</configuration>

　　Test

using System;
using System.Data;
using System.Configuration;
using System.Collections;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;

using Pattern.Provider;

public partial class Provider : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        Response.Write(Message.Insert(new MessageModel("插入", DateTime.Now)));
        Response.Write("<br />");
        Response.Write(Message.Get()[0].Message + " " + Message.Get()[0].PublishTime.ToString());
    }
}

　　运行结果
　　True
　　SQL方式，连接字符串是server=.;database=db;uid=sa;pwd=sa 2007-1-22 8:21:44